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This thesis examines the degradation pathways of chlorophyll in the Black Sea
water column and sediments. Measurements are made of total chlorophyll in sediment
traps from two locations and depths in the water column, and at two locations in surface
sediments. Individual chlorophyll degradation products are also identified. This data is
used to construct a mass balance of chlorophyll production and sedimentation showing the
major pathways for chlorophyll loss and the ultimate sedimentary sinks. The distribution
of chlorophyll degradation products is also analyzed down core and related to
environmental changes in the Black Sea
Several new sinks for chlorophyll degradation products are identified. Steryl esters
of pyropheophorbide-a are identified in sediment trap and sediment samples. It is thought
that these compounds are formed during grazing. In sediment traps it is found that the
distribution of the sterols esterified to pyropheophorbide-a change with season and that the
sterols esterified are related to the distribution of sterols synthesized by the phytoplankton
living in the photic zone at the time of production. Analysis of pyropheophorbide-a steryl
esters in sediments shows the distribution of sterols to be quantitatively and qualitatively
more similar to the distribution of free sterols in sediment traps than in sediments. The
esterification of the sterols to pyropheophorbide-a apparently prevents the preferential
removal of 4-desmethylsterols relative to 4-methylsterols during degradation of the sterol
esters.
Chlorophyll degradation products which are incorporated into high molecular
weight material and material which is only accessible with strong acid are also identified.
The chlorophyll degradation products incorporated into these structures represent only a
few percent of the total structure. In the high molecular weight material, only phorbins
derived from chlorophyll-a are identifies, where as in the acid extractable material,
porphyrins are also identified. In surface sediments, the acid extractable chlorophyll
degradations products and the solvent extractable macromolecular chlorophyll degradation
products each comprise approximately 30% of total sedimentary chlorophyll degradation
products. The acid extractable chlorophyll degradation products are identified in sediment
trap samples, and evidence is presented for the occurrence of the solvent extractable
macromolecular chlorophyll degradation products in sediment trap samples.
Using data from sediment traps, sediments, and the literature, a mass balance of
chlorophyll flux, degradation, and accumulation in the Black Sea is presented. In the
photic zone, chlorophyll degradation products are either destroyed by photo-oxidation and
grazing, or they are transported into the anoxic water column in large, rapidly sinking
particles. Once the chlorophyll degradation products have reached the anoxic water
column, they survive to be deposited in the underlying sediments. As a comparison, 25
times more total organic carbon reaches the anoxic water column than does total phorbin,
but 75% of total organic carbon which reaches the anoxic water column is degraded, either
in the anoxic water column or in the very surface sediments. Though a larger percentage of
total organic carbon passes out of the photic zone, the phorbin macrocycle appears to be
more stable under anoxic conditions than is total organic carbon. The chlorophyll which
can be detected below the chemocline of the Black Sea in the form of chlorophyll
degradation products will survive to be deposited in surface sediments. Once in sediments,
chlorophyll degradation products are found in four different reservoirs: phorbin steryl
esters, free phorbins, solvent extractable macromolecular chlorophyll degradation products,
and acid extractable chlorophyll degradation products. Evidence for the occurrence of
porphyrins in surface Black Sea sediments is also presented.
The distribution of chlorophyll degradation products in Unit I Black Sea sediments
varies greatly with sediment depth. The concentration of total phorbin generally increases
with increasing burial depth, but the concentrations of the individual chlorophyll
degradation products vary in a manner which is both dissimilar to total phorbin and to each
other. No parent/daughter relationships for the chlorophyll degradation products are
indicated by the data. The distribution of sterols esterified to pyropheophorbide-a changes
with sediment depth with the largest qualitative changes occurring in strata where the total
phorbin concentration shows the largest quantitative changes. It is suggested that the
variations seen in the esterified sterols are related to changes in the phytoplankton
community over time. From the presented data, it is also suggested that total phorbin
concentration, normalized to total organic carbon, in Black Sea Unit I sediments is related
to paleoprimary production.
Several conclusions are drawn from the work presented in this thesis. There is
approximately 3 times more chlorophyll-derived phorbin in Black Sea sediments than can
be accounted for when considering only individual pheopigments, and therefore the
sedimentary degradation of chlorophyll is much more complex than previously thought. In
the anoxic sediments of the Black Sea, the total phorbin distribution can be accounted for
with organically extractable high molecular weight degradation products,
pyropheophorbide steryl esters, pheopigments, and acid extractable chlorophyll
degradation products. The sterol distribution in the pyropheophorbide steryl esters may
preserve the sterol distribution in surface waters as synthesized by the phytoplankton, and
pyropheophorbide steryl esters are preserved in sediments over the long term.

Description

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution November 1992

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